Limit analysis of 2-D and 3-D structures based on an ellipsoid yield criterion

In this paper, a nonlinear numerical technique is developed to calculate the limit load and failure mode of structures obeying an ellipsoid yield criterion by means of the kinematic limit theorem, nonlinear programming theory and displacement-based finite element method. Using an associated flow rule, a general yield criterion expressed by an ellipsoid equation can be directly introduced into the kinematic theorem of limit analysis. The yield surface is not linearized and instead a nonlinear purely kinematic formulation is obtained. The nonlinear formulation has a smaller number of constraints and requires less computational effort than a linear formulation. By applying the finite element method, the kinematic limit analysis with an ellipsoid yield criterion is formulated as a nonlinear mathematical programming problem subject to only a small number of equality constraints. The objective function corresponds to the dissipation power which is to be minimized and an upper bound to the plastic limit load of a structure can then be calculated by solving the minimum optimization problem. An effective, direct iterative algorithm has been developed to solve the resulting nonlinear programming formulation. The calculation is based purely on kinematically admissible velocities. The stress field does not need to be calculated and the failure mode of structures can be obtained. The proposed method can be used to calculate the bearing capacity of clay soils in a direct way. Some examples are given to illustrate the validity and effectiveness of the proposed method.     

西藏搭格架热泉型铯矿床地质特征及形成时代

文章在野外第四纪地质与地貌调查的基础上,系统研究了搭格架矿床的地质特征,并以U系法的全溶法和等时线法为主要手段,查明了矿床的成矿时代。根据泉华在野外的分布特征,将其分为6套。第Ⅰ套为灰白色钙华;第Ⅱ～Ⅵ套为硅华,主要矿物为胶状和粒状蛋白石。硅华在长马曲河流阶地的位置分别为:第Ⅱ套,T5;第Ⅲ套,T4;第Ⅳ套,T3;第Ⅴ套,T2;第Ⅵ套,T2。泉华形成于5个阶段:403～202kaB.P.;99kaB.P.;39～25kaB.P.;17～4kaB.P.;现代。     

Polygonal faulting and petroleum geological significance of Qn1 formation in the Sanzhao Sag,Songliao BasinEI北大核心CSCD

Numerous small-scaled, extensional faults occur densely in the mudstone layer of the Qingshankou 1st member (Qn1) in Songliao Basin. Its attribute and genetic mechanism are highly concerned but still controversy. The high-resolution 3D seismic datasets of a new block that covers most of the Sanzhao sag of Songliao Basin is studied., The characteristics of the profile and plane geometry of the extensional fracture system in the Sanzhao sag were depicted in detail through fine seismic interpretation using coherent technology. Based o the results, we proposed that: (1) The extensional fault system is mainly polygonal fault, which probably resulted from rupture pressure related to the episodic hydrocarbon-expulsion from the mudstone of Qn1. However, the tectonic inversion in Songliao Basin may also contribute to the formation and development of the polygonal fault system; (2) The polygonal faults were mainly formed by the end of deposition period of the Nenjiang Formation, and some experienced vertical direction propagation caused by the strong tectonic inversion at the end of deposition period of the Mingshui Formation; (3) The polygonal faults connect the Qn1 hydrocarbon source rock with the underlying Fuyang oil layer and the overlying Putaohua oil layer, and thus are important migration channels for the oil and gas accumulation; (4) Until the end of deposition period of the Nenjiang Formation, hydrocarbon generated in the Qn1 mainly migrated downward into the underlying Fuyang oil layer via the small polygonal faults rather than upwards to the Putaohua oil layer. On the contrary, by the end of deposition period of the Mingshui Formation, the oils from Qn1 source rock can migrate either downward into the Fuyang oil layer or upward into the Putaohua oil layer. It is noteworthy that the vertical propagation of the polygonal faults induced by the tectonic inversion might also damage the Fuyang accumulations formed earlier. ©, 2015, Science Press. All right reserved.     

Salt tectonics in pull-apart basins with application to the Dead Sea Basin

The Dead Sea Basin displays a broad range of salt-related structures that developed in a sinistral strike-slip tectonic environment: en échelon salt ridges, large salt diapirs, transverse oblique normal faults, salt walls and rollovers. Laboratory experiments are used to investigate the mechanics of salt tectonics in pull-apart systems. The results show that in an elongated pull-apart basin the basin fill, although decoupled from the underlying basement by a salt layer, remains frictionally coupled to the boundary. The basin fill, therefore, undergoes a strike-slip shear couple that simultaneously generates en échelon fold trains and oblique normal faults, trending mutually perpendicular. According to the orientation of basin boundaries, sedimentary cover deformation can be dominantly contractional or extensional, at the extremities of pull-apart basins forming either folds and thrusts or normal faults, respectively. These guidelines, applied to the analysis of the Dead Sea Basin, show that the various salt-related structures form a coherent set in the frame of a sinistral strike-slip shearing deformation of the sedimentary basin fill.     

The Bar and Spiral Structure Legacy (BeSSeL) survey: Mapping the Milky Way with VLBI astrometry

Astrometric Very Long Baseline Interferometry (VLBI) observations of maser sources in the Milky Way are used to map the spiral structure of our galaxy and to determine fundamental parameters such as the rotation velocity (Θ₀) and curve and the distance to the Galactic center (R₀). Here, we present an update on our first results, implementing a recent change in the knowledge about the Solar motion. It seems unavoidable that the IAU recommended values for R₀ and Θ₀ need a substantial revision. In particular the combination of 8.5 kpc and 220 km s^–1 can be ruled out with high confidence. Combining the maser data with the distance to the Galactic center from stellar orbits and the proper motion of Sgr A* gives best values of R₀ = 8.3 ± 0.23 kpc and Θ₀ = 239 or 246±7 km s^–1, for Solar motions of V_⊙ = 12.23 and 5.25 km s^–1, respectively. Finally, we give an outlook to future observations in the Bar and Spiral Structure Legacy (BeSSeL) survey (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Silicon isotope fractionation between rice plants and nutrient solution and its significance to the study of the silicon cycle

The silicon isotope fractionation between rice plant and nutrient solution was studied experimentally. Rice plants were grown to maturity with the hydroponic culture in a naturally lit glasshouse. The nutrient solution was sampled for 14 times during the whole rice growth period. The rice plants were collected at various growth stages and different parts of the plants were sampled separately. The silica contents of the samples were determined by the gravimetric method and the silicon isotope compositions were measured using the SiF₄ method.In the growth process, the silicon content in the nutrient solution decreased gradually from 16 mM at starting stage to 0.1-0.2 mM at harvest and the amount of silica in single rice plant increased gradually from 0.00013 g at start to 4.329 g at harvest. Within rice plant the SiO₂ fraction in roots reduced continuously from 0.23 at the seedling stage, through 0.12 at the tiller stage, 0.05 at the jointing stage, 0.023 at the heading stage, to 0.009 at the maturity stage. Accordingly, the fraction of SiO₂ in aerial parts increased from 0.77, through 0.88, 0.95, 0.977, to 0.991 for the same stages. The silicon content in roots decreased from the jointing stage, through the heading stage, to the maturity stage, parallel to the decrease of silicon content in the nutrient solution. At the maturity stage, the silicon content increased from roots, through stem and leaves, to husks, but decreased drastically from husks to grains. These observations show that transpiration and evaporation may play an important role in silica transportation and precipitation within rice plants.It was observed that the δ³⁰Si of the nutrient solution increased gradually from −0.1‰ at start to 1.5‰ at harvest, and the δ³⁰Si of silicon absorbed by bulk rice plant increased gradually from −1.72‰ at start to −0.08‰ at harvest, reflecting the effect of the kinetic silicon isotope fractionation during silicon absorption by rice plants from nutrient solutions. The calculated silicon isotope fractionation factor between the silicon instantaneously absorbed by rice roots and the silicon in nutrient solution vary from 0.9983 at start to 0.9995 at harvest, similar to those reported for bamboo, banana and diatoms in direction and extent. In the maturity stage, the δ³⁰Si value of rice organs decreased from −1.33‰ in roots to −1.98‰ in stem, and then increased through −0.16‰ in leaves and 1.24‰ in husks, to 2.21‰ in grains. This trend is similar to those observed in the field grown rice and bamboo.These quantitative data provide us a solid base for understanding the mechanisms of silicon absorption, transportation and precipitation in rice plants and the role of rice growth in the continental Si cycle.     

Oxygen self-diffusion “fast-paths” in titanite single crystals and a general method for deconvolving self-diffusion profiles with “tails”

Like most other minerals, titanite rarely if ever forms perfect crystals. In addition to the point defects that might affect lattice diffusion, there may be extended line- or planar defects along which fast diffusion could occur. During the course of an experimental study of oxygen lattice diffusion in titanite, we found that almost all of the ¹⁸O uptake profiles produced in natural titanite crystals departed from the complementary error function solution expected for simple lattice diffusion with a constant surface concentration. Instead, they exhibited “tails” extending deeper into the samples than expected for simple lattice diffusion. The purpose of this contribution is to report on these features—described as “fast-paths” for oxygen diffusion—and outline a method for coping with them in extracting information from diffusion profiles.For both dry and hydrothermal experiments in which the “fast paths” are observed, ¹⁸O was used as the diffusant. In dry experiments, the source material was ¹⁸O-enriched SiO₂ powder, while ¹⁸O-enriched water was used for the hydrothermal experiments. Diffusive uptake profiles of ¹⁸O were measured in all cases by nuclear reaction analysis (NRA) using the ¹⁸O (p,α)¹⁵N reaction [see Zhang X. Y., Cherniak D. J., and Watson E. B. (2006) Oxygen diffusion in titanite: lattice and fast-path diffusion in single crystals. Chem. Geol.235 105-123].In our experiments, different sizes of “tails” (with varying ¹⁸O concentrations) were observed. Theoretically, under the same temperature and pressure conditions, the sizes of tails should be affected by two factors: the diffusion duration and the defect density. For the same experiment duration, the higher the defect density, the larger the “tail”; for the same defect densities, the longer the diffusion duration, the larger the “tail.”The diffusion “tails” could be a result of either planar defects or one-dimensional “pipe” diffusion. AFM imaging of HF etched titanite surfaces confirmed that the etched features might be caused by either parallel planar defects or parallel pipe defects, but could not differentiate between these possibilities. Through theoretical calculations simulating the tailed diffusion profiles using reasonable assumptions of lattice diffusivities and fast-path diffusivities, and comparing these with tail features measured in our samples, it can be concluded that the “tails” observed in our experiments are caused by planar defects rather than pipe defects.A new method was developed for separating the “fast-path” contribution from the overall composite diffusion profile consisting of both “fast-path” and lattice diffusion. Through this process, the lattice diffusion coefficient could be determined, which is required to analyze the tail. The oxygen diffusion rates in the fast-paths were obtained by traditional graphical analysis methods, using the Whipple-Le Claire equation (for 2-D defects) assuming that the width of the fast-path is 1 nm. Two Arrhenius relations were obtained for the fast-path diffusion phenomenon, one for experiments under dry conditions, and the other for hydrothermal conditions:

     

Application of multivariate statistical approach to identify heavy metal sources in sediment and waters: a case study in Yangzhong,China

Multivariate statistical approach is used to identify the sources of heavy metals (Bi, Cd, Co, Cr, Mn, Pb, U, V, and Zn) in surface water and freshly deposited riverine sediment samples in Yangzhong city, China. The metal concentration data for the water and sediment samples are reported in terms of basic statistical parameters and metal-to-metal correlations. In both surface water and sediment samples, significant correlations are observed between some metals. Principal component analysis and cluster analysis distinguishes factors of lithogenic and anthropogenic origin. Bismuth, Cd, Co, and Pb (Co only for water samples) contents are controlled by the regional lithogenic high background factor; Co, Mn, U, and V (Co only for sediment samples) are interpreted to be mainly inherited from soil parent materials, while Cr, Zn, and Mn in the two kinds of samples are recognized as the tracer of industrial pollution. Obvious similarity between factor loadings of the two kinds of samples is observed, evidencing that metal variability in the two kinds of samples is controlled by the same sources. Statistical analysis agrees with discussion based on background value and field survey of point-source pollutant affected sediment, making this statistical discussion more convincing.     

大青山山前断裂带中、晚全新世活动的发现

大青山山前断裂带位于大青山南麓,为河套断裂系的重要组成部分。该断裂带控制大青山山体的抬升和呼和断陷的形成,地貌特征十分明显,第四纪断层十分发育。大青山山前断裂带第四纪强烈活动,但有关全新世活动的确凿证据未能肯定,亦未见有报道。近年来,我们在包头东河区至沙尔沁一带,发现含全新世文化层的坡、洪积地层多处被错断,证实大青山山前断裂带中、晚全新世活动仍较显著,这对研究河套地区可能存在古地震遗迹提供了重要的线索。     

Wave Attenuation at a Salt Marsh Margin: A Case Study of an Exposed Coast on the Yangtze Estuary

To quantify wave attenuation by (introduced) Spartina alterniflora vegetation at an exposed macrotidal coast in the Yangtze Estuary, China, wave parameters and water depth were measured during 13 consecutive tides at nine locations ranging from 10 m seaward to 50 m landward of the low marsh edge. During this period, the incident wave height ranged from <0.1 to 1.5 m, the maximum of which is much higher than observed in other marsh areas around the world. Our measurements and calculations showed that the wave attenuation rate per unit distance was 1 to 2 magnitudes higher over the marsh than over an adjacent mudflat. Although the elevation gradient of the marsh margin was significantly higher than that of the adjacent mudflat, more than 80% of wave attenuation was ascribed to the presence of vegetation, suggesting that shoaling effects were of minor importance. On average, waves reaching the marsh were eliminated over a distance of ∼80 m, although a marsh distance of ≥100 m was needed before the maximum height waves were fully attenuated during high tides. These attenuation distances were longer than those previously found in American salt marshes, mainly due to the macrotidal and exposed conditions at the present site. The ratio of water depth to plant height showed an inverse correlation with wave attenuation rate, indicating that plant height is a crucial factor determining the efficiency of wave attenuation. Consequently, the tall shoots of the introduced S. alterniflora makes this species much more efficient at attenuating waves than the shorter, native pioneer species in the Yangtze Estuary, and should therefore be considered as a factor in coastal management during the present era of sea-level rise and global change. We also found that wave attenuation across the salt marsh can be predicted using published models when a suitable coefficient is incorporated to account for drag, which varies in place and time due to differences in plant characteristics and abiotic conditions (i.e., bed gradient, initial water depth, and wave action).